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Hack -> compromise

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hacker -> intruder (couldn't desired between this and 'cracker')
config -> configuration
sorted crossreferences
spell checked

Overall very good content, but we need one of our wordsmiths to change the
tone to match the CSRG manpages.
This commit is contained in:
David E. O'Brien 1998-12-25 23:39:01 +00:00
parent 257bf67682
commit 6ac7e896c3
1 changed files with 51 additions and 52 deletions
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103
share/man/man7/security.7
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@ -10,7 +10,7 @@
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" must display the following acknowledgment:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
@ -30,7 +30,7 @@
.\" SUCH DAMAGE.
.\"
.\" @(#)security.1 8.2 (Berkeley) 12/30/93
.\" $Id: security.7,v 1.1 1998/12/20 20:12:17 dillon Exp $
.\" $Id: security.7,v 1.2 1998/12/22 19:02:51 dillon Exp $
.\"
.Dd December 30, 1993
.Dt SECURITY 7
@ -45,10 +45,10 @@ While all
.Bx
systems are inherently multi-user capable, the job of building and
maintaining security mechanisms to keep those users 'honest' is probably
one of the single largest undertakings of the sysad. Machines are
one of the single largest undertakings of the sysadmin. Machines are
only as secure as you make them, and security concerns are ever competing
with the human necessity for convenience. UNIX systems,
in general, are capable of running a huge number of simultanious processes
in general, are capable of running a huge number of simultaneous processes
and many of these processes operate as servers - meaning that external entities
can connect and talk to them. As yesterday's mini-computers and mainframes
become today's desktops, and as computers become networked and internetworked,
@ -61,9 +61,9 @@ Denial of service attacks
.It
User account compromises
.It
Root Hacks through accessible servers
Root compromise through accessible servers
.It
Root Hacks via user accounts
Root compromise via user accounts
.El
.Pp
A denial of service attack is an action that deprives the machine of needed
@ -78,13 +78,13 @@ attacks are harder to deal with. A spoofed-packet attack, for example, is
nearly impossible to stop short of cutting your system off from the internet.
.Pp
A user account compromise is even more common then a D.O.S. attack. Many
sysops still run standard telnetd, rlogind, rshd, and ftpd servers on their
sysadmins still run standard telnetd, rlogind, rshd, and ftpd servers on their
machines. These servers, by default, do not operate over encrypted
connections. The result is that if you have any moderate-sized user base,
one or more of your users logging into your system from a remote location
(which is the most common and convenient way to login to a system) will
have his or her password sniffed. The attentive system admin will analyze
his remote access logs occassionally looking for suspicious source addresses
his remote access logs occasionally looking for suspicious source addresses
even for successful logins.
.Pp
One must always assume that once an attacker has access to a user account,
@ -116,7 +116,7 @@ Securing the password file
.It
Securing the kernel core, raw devices, and filesystems
.It
Checking file integrity: binaries, config files, and so forth
Checking file integrity: binaries, configuration files, and so forth
.It
Paranoia
.El
@ -141,29 +141,29 @@ staff accounts to the wheel group (in /etc/group). The staff members placed
in the wheel group are allowed to 'su' to root. You should never give staff
members native wheel access via their entry in the password file... put staff
in a 'staff' group or something and only add those that really need root to
the wheel group. Unfortunately the wheel mechanism still allows a hacker to
break root if the hacker has gotten hold of your password file - he need only
the wheel group. Unfortunately the wheel mechanism still allows an intruder to
break root if the intruder has gotten hold of your password file - he need only
break the root password and the password of one of the staff accounts that
happens to be in the wheel group. So while the wheel mechanism is useable,
happens to be in the wheel group. So while the wheel mechanism is usable,
it isn't much safer then not having a wheel group at all.
.Pp
An indirect way to secure the root account is to secure your staff accounts
by using an alternative login access method and *'ing out the crypted password
for the staff accounts. This way a hacker may be able to steal the password
for the staff accounts. This way an intruder may be able to steal the password
file but will not be able to break into any staff accounts (or, indirectly,
root, even if root has a crypted password associated with it). Staff members
get into their staff accounts through a secure login mechanism such as
kerberos(1) or ssh(1) (see /usr/ports/security/ssh) using a private/public
keypair. When you use something like kerberos you generally must secure
key pair. When you use something like kerberos you generally must secure
the machines which run the kerberos servers and your desktop workstation.
When you use a public/private keypair with ssh, you must generally secure
When you use a public/private key pair with ssh, you must generally secure
the machine you are logging in FROM (typically your workstation), but you can
also add an additional layer of protection to the keypair by password
protecting the keypair when you create it with ssh-keygen(1). Being able
to *-out the passwords for staff accounts also guarentees that staff members
also add an additional layer of protection to the key pair by password
protecting the key pair when you create it with ssh-keygen(1). Being able
to *-out the passwords for staff accounts also guarantees that staff members
can only login through secure access methods that you have setup. You can
thus force all staff members to use secure, encrypted connections for
all their sessions which closes an important hole used by many hackers: That
all their sessions which closes an important hole used by many intruders: That
of sniffing the network from an unrelated, less secure machine.
.Pp
The more indirect security mechanisms also assume that you are logging in
@ -175,7 +175,7 @@ at all, and you should run a password-protected screen blanker.
Of course, given physical access to
a workstation an attacker can break any sort of security you put on it.
This is definitely a problem that you should consider but you should also
consider the fact that the vast majority of breakins occur remotely, over
consider the fact that the vast majority of break-ins occur remotely, over
a network, from people who do not have physical access to your workstation or
servers.
.Pp
@ -197,7 +197,7 @@ running an old version of imapd or popper is like giving a universal root
ticket out to the entire world. Never run a server that you have not checked
out carefully. Many servers do not need to be run as root. For example,
the ntalk, comsat, and finger daemons can be run in special user 'sandboxes'.
A sandbox isn't perfect unless you go to a hellofalot of trouble, but the
A sandbox isn't perfect unless you go to a large amount of trouble, but the
onion approach to security still stands: If someone is able to break in
through a server running in a sandbox, they still have to break out of the
sandbox. The more layers the attacker must break through, the lower the
@ -218,30 +218,30 @@ There are a number of other servers that typically do not run in sandboxes:
sendmail, popper, imapd, ftpd, and others. There are alternatives to
some of these, but installing them may require more work then you are willing
to put (the convenience factor strikes again). You may have to run these
servers as root and rely on other mechanisms to detect breakins that might
servers as root and rely on other mechanisms to detect break-ins that might
occur through them.
.Pp
The other big potential root hole in a system are the suid-root and sgid
binaries installed on the system. Most of these binaries, such as rlogin,
reside in /bin, /sbin, /usr/bin, or /usr/sbin. While nothing is 100% safe,
the system-default suid and sgid binaries can be considered reasonably safe.
Still, root holes are occassionaly found in these binaries. A root hole
Still, root holes are occasionally found in these binaries. A root hole
was found in Xlib in 1998 that made xterm (which is typically suid) vulnerable.
It is better to be safe then sorry and the prudent sysadmin will restrict suid
binaries that only staff should run to a special group that only staff can
access, and get rid of (chmod 000) any suid binaries that nobody uses. A
server with no display generally does not need an xterm binary. Sgid binaries
can be almost as dangerous. If a hacker can break an sgid-kmem binary the
hacker might be able to read /dev/kmem and thus read the crypted password
file, potentially compromising any passworded account. A hacker that breaks
can be almost as dangerous. If an intruder can break an sgid-kmem binary the
intruder might be able to read /dev/kmem and thus read the crypted password
file, potentially compromising any passworded account. An intruder that breaks
the tty group can write to almost user's tty. If a user is running a terminal
program or emulator with a talk-back feature, the hacker can potentially
program or emulator with a talk-back feature, the intruder can potentially
generate a data stream that causes the user's terminal to echo a command, which
is then run as that user.
.Sh SECURING USER ACCOUNTS
.Pp
User accounts are usually the most difficult to secure. While you can impose
draconian access restrictions on your staff and *-out their passwords, you
Draconian access restrictions on your staff and *-out their passwords, you
may not be able to do so with any general user accounts you might have. If
you do have sufficient control then you may win out and be able to secure the
user accounts properly. If not, you simply have to be more vigilant in your
@ -253,7 +253,7 @@ password.
The only sure fire way is to *-out as many passwords as you can and
use ssh or kerberos for access to those accounts. Even though the
crypted password file (/etc/spwd.db) can only be read by root, it may
be possible for a hacker to obtain read access to that file even if the
be possible for a intruder to obtain read access to that file even if the
attacker cannot obtain root-write access.
.Pp
Your security scripts should always check for and report changes to
@ -263,18 +263,18 @@ the password file (see 'Checking file integrity' below).
If an attacker breaks root he can do just about anything, but there
are certain conveniences. For example, most modern kernels have a
packet sniffing device driver built in. Under FreeBSD it is called
the 'bpf' device. A hacker will commonly attempt to run a packet sniffer
on a compromised machine. You do not need to give the hacker the
the 'bpf' device. A intruder will commonly attempt to run a packet sniffer
on a compromised machine. You do not need to give the intruder the
capability and most systems should not have the bpf device compiled in.
Unfortunately, there is another kernel feature called the Loadable Kernel
Module interface. An enterprising hacker can use an LKM to install
Module interface. An enterprising intruder can use an LKM to install
his own bpf device or other sniffing device on a running kernel. If you
do not need to use the module loader, turn it off in the kernel config
do not need to use the module loader, turn it off in the kernel configuration
with the NO_LKM option.
.Pp
But even if you turn off the bpf device, and turn off the module loader,
you still have /dev/mem and /dev/kmem to worry about. For that matter,
the hacker can still write raw devices. To avoid this you have to run
the intruder can still write raw devices. To avoid this you have to run
the kernel at a higher secure level... at least securelevel 1. The securelevel
can be set with a sysctl on the kern.securelevel variable. Once you have
set the securelevel to 1, write access to raw devices will be denied and
@ -311,15 +311,15 @@ binaries), then emails each staff member a daily report of differences.
.Pp
Another way to do this sort of check is to NFS export the major filesystems
from every other machine to the security machine. This is somewhat more
network intensive but also virtually impossible for a hacker to detect
network intensive but also virtually impossible for an intruder to detect
or spoof.
.Pp
A good security script will also check for changes to user and staff members
access configuration files: .rhosts, .shosts, .ssh/authorized_keys, and
so forth... files that might fall outside the pervue of the MD5 check.
so forth... files that might fall outside the prevue of the MD5 check.
.Pp
A good security script will check for suid and sgid binaries on all
filesystems and report their absolute existance as well as a diff against
filesystems and report their absolute existence as well as a diff against
the previous report or some baseline (say, make a baseline once a week).
While you can turn off the ability to run suid and sgid binaries on certain
filesystems through the 'nosuid' option in fstab/mount, you cannot turn this
@ -328,18 +328,18 @@ If you have a huge amount of user disk space, though, it may be useful to
disallow suid binaries and devices ('nodev' option) on the user partitions
so you do not have to scan them for such. I would scan them anyway, though,
at least once a week, since the object of this onion layer is detection of
a breakin.
a break-in.
.Pp
Process accounting (see accton(1)) is a relatively low-overhead feature of
the operating system which I recommend using as a post-breakin evaluation
mechanism. It is especially useful in tracking down how a hacker has
the operating system which I recommend using as a post-break-in evaluation
mechanism. It is especially useful in tracking down how an intruder has
actually broken root on a system, assuming the file is still intact after
the breakin occurs.
the break-in occurs.
.Pp
Finally, security scripts should process the log files and the logs themselves
should be generated in as secured a manner as possible - remote syslog can be
very useful. A hacker tries to cover his tracks, and log files are critical
to the sysadmin trying to track down the time and method of the initial breakin.
very useful. An intruder tries to cover his tracks, and log files are critical
to the sysadmin trying to track down the time and method of the initial break-in.
.Sh PARANOIA
.Pp
A little paranoia never hurts. As a rule, a sysadmin can add any number
@ -348,7 +348,7 @@ can add security features that do effect convenience with some added
thought.
.Sh SPECIAL SECTION ON D.O.S. ATTACKS
.Pp
This section covers Dential of Service attacks. A DOS attack is typically
This section covers Denial of Service attacks. A DOS attack is typically
a packet attack. While there isn't much you can do about modern spoofed
packet attacks that saturate your network, you can generally limit the damage
by ensuring that the attacks cannot take down your servers.
@ -362,7 +362,7 @@ Kernel Route Cache
.El
.Pp
A common DOS attack is against a forking server that attempts to cause the
server to eat processes, file descirptors, and memory until the machine
server to eat processes, file descriptors, and memory until the machine
dies. Inetd (see inetd(8)) has several options to limit this sort of attack.
It should be noted that while it is possible to prevent a machine from going
down it is not generally possible to prevent a service from being disrupted
@ -393,7 +393,7 @@ feature of tcpwrappers for this reason.
It is a very good idea to protect internal services from external access
by firewalling them off at your border routers. The idea here is to prevent
saturation attacks from outside your LAN, not so much to protect internal
services from root network-based root hacks. Always configure an exclusive
services from root network-based root compromise. Always configure an exclusive
firewall, i.e. 'firewall everything *except* ports A, B, C, D, and M-Z'. This
way you can firewall off all of your low ports except for certain specific
services such as named (if you are primary for a zone), ntalkd, sendmail,
@ -457,19 +457,18 @@ table from attack.
.Sh SEE ALSO
.Pp
.Xr ssh 1 ,
.Xr sshd 1 ,
.Xr kerberos 1 ,
.Xr accton 1 ,
.Xr xdm 1 ,
.Xr syslogd 1 ,
.Xr chflags 1 ,
.Xr find 1 ,
.Xr kerberos 1 ,
.Xr md5 1 ,
.Xr ssh 1 ,
.Xr sshd 1 ,
.Xr syslogd 1 ,
.Xr xdm 1 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
manual page was originally written by Matthew Dillon and first appeared
in FreeBSD-3.0.1, December 1998.